The process of adding water to calcium oxide to produce calcium hydroxide is referred to as hydration process or lime slaking. The hydration process, when done with just the right amount of water for chemical reaction, is called dry hydration process. The product of dry hydration process is Hydrated Lime which is a dry powder and in which the oxides are converted to hydroxides. Dry hydrated lime is generally produced from high calcium quicklime and contains 72 % to 74 % calcium oxide with 23 % to 24 % chemically combined water. The hydration reaction is chemically simple but it is strongly exothermic, with a heat generation of around 276 kcal/kg of CaO. The reaction is given below. CaO + H2O = Ca(OH)2 + heat Hence, 56 kilograms of CaO combines with 18 kilograms of water to produce 74 kilograms of Ca(OH)2. The ratio of hydroxide to CaO is 1.32. As per stoichiometric composition, calcium hydroxide contains 75.7 % CaO and 24.3 % H2O.